TACI, a member of the TNF superfamily, is known to function as a key regulator of B cells. TACI has two cysteine-rich domains (hereinafter, referred to as “CRDs”) in its extracellular region and binds to two ligands (APRIL and BAFF). TACI lacking N-terminal CRD as a result of alternative splicing is also found in nature. Reportedly, TACI C-terminal CDR (TACI_d2) alone exhibits binding activity against both the ligands that is equivalent to the binding activity of the whole extracellular region (TACI_d1d2) (See International Publication No. WO 2006/052493; Melissa A. Starovasnik, J. Biol. Chem., vol. 280 (No. 8), pp. 7218-7227 (2005).)
However, whether or not TACI_d2 or a variant thereof exhibits high binding activity against a molecule other than the endogenous ligands has not yet been revealed.